1. Field of the Invention
The field of the invention relates generally to Radio Frequency Identification (RFID) systems and more particularly to systems and methods for synchronizing a plurality of RFID interrogators in a theatre of operation.
2. Background of the Invention
FIG. 1 illustrates a basic RFID system 100. A basic RFID system 100 comprises three components: an antenna or coil 104, an interrogator 102 with decoder 112, and a transponder, or RF tag 106 which is often electronically programmed with unique information. Antenna 104 emits radio signals 110 to activate and read and write data to tag 106. Antenna 104 is the conduit between tag 106 and interrogator 102, which controls data acquisition and communication. Antennas 104 are available in a variety of shapes and size, for example, in certain embodiments they can be built into a door frame to receive tag data from persons or things passing through the door. In other embodiments, antennas 104 can, for example, be mounted on an interstate toll booth to monitor traffic passing by on a freeway. Further, depending on the embodiments, the electromagnetic field, i.e., radio signal 110, produced by an antenna 104 can be constantly present when, e.g., multiple tags 106 are expected continually. If constant interrogation is not required, then radio signal 110 can, for example, be activated by a sensor device.
Often antenna 104 is packaged with interrogator 102. A conventional interrogator 102 can emit radio signals 110 in ranges of anywhere from one inch to 100 feet or more, depending upon the power output and the radio frequency used. When an RFID tag 106 passes through an electromagnetic zone associated with radio signal 106, it detects radio signal 106, which can comprise an activation signal. In some embodiments, interrogators can comprise multiple antenna, though typically only one transmits at a time.
RFID tags 106 come in a wide variety of shapes and sizes. Animal tracking tags, for example, inserted beneath the skin of an animal, can be as small as a pencil lead in diameter and one-half inch in length. Tags 106 can be screw-shaped for insertion, e.g., in order to identify trees or wooden items, or credit-card shaped for use in access applications. Anti-theft hard plastic tags that include RFID tags 106 can be attached to merchandise in stores. Heavy-duty RFID tags can be used to track intermodal containers, heavy machinery, trucks, and/or railroad cars for maintenance and/or tracking purposes. A multitude of other uses and applications also exists, and many more will come into being in the future.
RFID tags 106 are categorized as either active or passive. Active RFID tags 106 are powered by an internal battery and are typically read/write, i.e., tag data can be rewritten and/or modified. An active tag's memory size varies according to application requirements. For example, some systems operate with up to 1 MB of memory. In a typical read/write RFID work-in-process system, a tag 106 might give a machine a set of instructions, and the machine would then report its performance to tag 106. This encoded data would then become part of the tagged part's history. The battery-supplied power of an active tag 106 generally gives it a longer read and write range. The trade off is greater size, greater cost, and a limited operational life.
Passive RFID tags 106 operate without a separate external power source and obtain operating power generated from radio signal 110. Passive tags 106 are consequently much lighter than active tags 106, less expensive, and offer a virtually unlimited operational lifetime. The trade off is that they have shorter read ranges than active tags 106 and require a higher-powered interrogator 102. Read-only tags are typically passive and are programmed with a unique set of data, usually 32 to 128 bits, that cannot be modified. Read-only tags 106 often operate as a license plate into a database, in the same way as linear barcodes reference a database containing modifiable product-specific information. Not all passive tags 106 are read-only tags.
RFID systems are also distinguishable by their frequency ranges. Low-frequency, e.g., 30 KHz to 500 KHz, systems have short reading ranges and lower system costs. They are commonly used in security access, asset tracking, and animal identification applications. High-frequency, e.g., 850 MHz to 950 MHz and 2.4 GHz to 2.5 GHz, systems offer long read ranges, e.g., greater than 90 feet, high reading speeds, and are used for such applications as railroad car tracking and automated toll collection, however, the higher performance of high-frequency RFID systems 100 incurs higher system costs.
The significant advantage of all types of RFID systems 100 is the noncontact, non-line-of-sight nature of the technology. Tags 106 can be read through a variety of substances such as snow, fog, ice, paint, crusted grime, and other visually and environmentally challenging conditions, where barcodes or other optically read technologies cannot typically be used. RFID tags 106 can also be read in challenging circumstances at high speeds, often responding in less than 100 milliseconds. RFID has become indispensable for a wide range of automated data collection and identification applications that would not be possible otherwise.
A conventional RFID interrogator 102 comprises an RF transceiver 106 and a decoder 112. Decoder 112 can, for example, be a micro controller or other processing circuit configured to carryout the required functions. Often, decoder 112 is interfaced with memory 114. Firmware instructions used by decoder 112 to control the operation of interrogator 102 can be stored in memory 114, along with RFID instructions that can be communicated to RFID tag 106 and can be used to control acquisition of information from RFID tags 106. Memory 114 can, depending on the embodiment, comprise one or more memory circuits.
FIG. 2 shows an example transmission operation of an RFID interrogator. Graph 200 shows a transmission of the RFID interrogator when no data is transmitted. At the start of each frame 202, interrogator 102 can be configured to transmit frame synchronization pulses 204, which can have a much shorter width than the period associated with frame 202. RFID interrogator 102 can transmit data to RFID tag 106 by modifying the frame synchronization pulses, for instance by doubling the pulses to represent a binary “zero” and tripling the synchronization pulses to represent a binary “one.” Graph 220 shows an example of such a transmission method by an RFID interrogator. Double pulses 222 and 230, which comprise two pulses sent within a short period compared to the frame period; represent the transmission of a “zero.” Triple pulse 226, which comprise three pulses sent within a short period compared to the frame period, represent the transmission of a “one.” Remaining single pulses 224 and 228 do not represent data and synchronize the associated frames.
Another method of modifying frame synchronization pulses used by RFID interrogators is to use wider pulses to represent a “zero” and still wider pulses to represent a “one.” Graph 240 shows an example of such a transmission method. The “wider” pulses 242 and 250, which are still short compared to the frame period, represent the transmission of a “zero.” The “widest” pulse 246, which is still short compared to the flame period but wider than pulses 242 and 250, represent the transmission of a “one.” The remaining “normal” width pulses 244 and 248 do not represent data and synchronize the associated frames 202.
Graphs 220 and 240 illustrate just two possible examples of communication protocols that can be used to facilitate transmission of data in system 100.
In response to interrogation signals from the interrogator 102, RFID tags 106 can be configured to respond in the second half of frames 202. Furthermore, in many embodiments of an RFID interrogation system 100 both tags 106 and interrogator 102 operate in the same frequency range. The synchronization pulses, whether “normal” or modified to carry data, can serve two additional purposes. First, the pulses can be used to define the boundaries of frames 202 so the tags 106 can respond at the appropriate time. Second, the pulses supply power for passive RFID tags 106.
FIG. 3 depicts an interrogation theatre 300 comprising a plurality of interrogators, of which interrogators 310 and 340 are shown for illustrative purposes. In addition, theatre 300 comprises a plurality of tags, of which tags 320, 322, and 344 are shown for illustrative purposes. Tags 320, 322, and 344 can for example, be similar to, or the same as, tag 106 described above. If allowed to operate independently, these readers can severely interfere with each other. To illustrate, in FIG. 3, RFID tags 320 and 322 are near interrogator 310, while RFID tag 344 is near interrogator 340. Temporally, interrogator 310 has just transmitted its request through its antenna 312 and is now awaiting a response signal from any nearby RFID tags. Because RFID tags 320 and 322 are near to interrogator 310, they respond with RFID signals 330 and 332, respectively; however, at approximately the same time, interrogator 340 wishes to interrogate RFID tags nearby such as RFID tag 344, by transmitting signal 346 through antenna 342. Since the responses 330 and 332 are on the same frequency as the interrogation signal 346, and interrogation signal 346 can be of greater power than signals 330 and 332, interrogator 310 may only detect the signal from interrogator 340 rather than from RFID tags 320 and 322.
FIG. 4 depicts the timing of the example given above. Graph 400 depicts interrogator 310 attempting to interrogate nearby RFID tags using the communications protocol illustrated by graph 220. RFID tag 320 responds and its RF output signal 330 is graphed over time in graph 410; however, with an unsynchronized RFID interrogator 340 also attempting to interrogate nearby RFID tags as depicted in graph 420, associated signal 346 can interfere with signal 330. As a result, antenna 312 sees the signal depicted in graph 430, where rather than seeing pulses 412 and 414 of signal 330 (graph 410), interrogator 310 is likely to see something like pulses 432 and 436 dominated by the influence of signal 346 (graph 420) of interrogator 340. As a result, interrogator 310 may interpret pulses 434 and 438 of interrogator 340 as coming from RFID tag 320, or interrogator 310 may just fail to code any signal or may receive corrupted information.